The present invention relates to a method for producing a lens, mirror, prism or similar plastic molding produced by injection molding and included in an optical device, e.g., a copier, laser printer, facsimile apparatus or similar image forming apparatus, and a method and an apparatus for producing the same. More particularly, the present invention is concerned with a method for producing a plastic molding having, e.g., mirror surfaces and a fine undulation pattern transferred thereto with high accuracy by injection molding.
For injection molding, it is a common practice to use a mold assembly including a mold surface forming a cavity having a preselected volume, a transfer surface formed on the mold surface for transferring a mirror surface to a molding, and a gate open at the mold surface and having a preselected opening area. Molten resin is injected into the cavity via the gate and then cooled. The resulting molding is taken out by opening the mold assembly. While such a molding, particularly a mirror, lens, prism or similar optical element, is required to have an accurate mirror surface and a uniform refractive index, the mirror surface needing a high surface accuracy is caused to sink because the molten resin contracts at the time of solidification.
Injection molding methods for solving the above problem are taught in, e.g., Japanese Patent Laid-Open Publication Nos. 3-128218, 8-234005, 3-151218, and 3-281213 (Prior Art 1 hereinafter). In prior Art 1, a non-transfer surface or mold surface facing a transfer surface formed with, e.g., a mirror surface is roughened, or surface treated for lowering wettability, or use is made of a porous material. Injection is stopped just before a cavity is filled up with molten resin. Then, the molten metal is solidified by cooling without any dwelling. As a result the roughened surface is caused to sink due to a difference in adhering force between the molten resin, the transfer surface, and the roughened surface. This prevents the mirror from sinking. Alternatively, an overflow portion for receiving excess molten resin is located outside of the cavity. When the overflow portion begins to be filled, injection is stopped. Then, the molten resin is solidified by cooling without any dwelling. This also allows the roughened surface to sink due to a difference in adhering force between the resin, the transfer surface, and the roughened surface.
An injection molding method disclosed in Japanese Patent Laid-Open Publication No. 2-175115 (Prior Art 2 hereinafter) injects molten metal into a cavity in which a porous member communicated to a compressed gas is provided on a mold surface contacting the non-transfer surface of a molding. While dwelling and cooling are under way after the injection of the molten resin, air is fed to the non-transfer surface of the molding via the porous member. With this method, it is possible to cause a side of a cylindrical thin lens to sink.
Japanese Patent Laid-Open Publication No. 6-304973 (Prior Art 3 hereinafter) proposes an injection molding method in which a non-transfer surface is communicated to the outside air via a vent hole. During an interval between the beginning and the end of injection of molten resin into a cavity, a pressure difference is generated between the transfer surface and the non-transfer surface of the resin. As a result, the non-transfer surface of the resin is caused to sink. Specifically, air is brought into contact with the molten resin other than the mirror transferred from the transfer surface via the vent hole and a bore communicated thereto, so that the cooling speed of the resin is lowered. At the same time, a preselected air pressure is fed to the vent hole in order to generate a preselected pressure difference between the mirror portion of the resin and the vent hole. This allows only the portion of the resin facing the vent hole to sink, i.e., prevents the mirror portion from sinking. In addition, because only the vent hole portion of the resin sinks, a molding can be produced by simple control over the amount of the resin to be injected into the cavity and without any strain being generated in the resin. The resulting molding is therefore free from an internal strain and provided with an accurate mirror surface.
Prior Art 3 further teaches that the vent hole may be communicated to a compressor so as to apply a preselected air pressure to the vent hole portion of the resin. With this configuration, it is possible to generate any desired pressure difference between the mirror surface portion and the vent hole portion of the resin, thereby causing the vent hole portion to sink. In addition, the pressure difference is readily adjustable in order to further enhance the accuracy of the mirror surface without any internal strain.
Japanese Patent Laid-Open Publication No. 6-31596 (Prior Art 4 hereinafter) teaches an injection molding method causing the non-transfer surface of resin to sink. In accordance with this method, the transfer surface of a mold heated to and held at a high temperature. The transfer surface side of the resin is heated to a high temperature until the injection of molten resin into a cavity ends.
However, Prior Art 1 relying on the roughened surface, surface treatment or porous material results in an expensive mold assembly. Moreover, stopping the injection just before the cavity is filled up with the molten metal is extremely difficult. Should the timing for stopping the injection be deviated, the relation in adhering force between the transfer surface and the roughened surface would be inverted and would thereby cause the mirror surface to sink or result in short resin. In addition, because sinking cannot be provided with directionality and because setting the molding conditions is difficult, the configuration of the molding is critically limited. The filling of the molten resin may be stopped at any time lying in a broader range. However, the overflow portion formed integrally with the molding must be removed by an extra step. Moreover, should the opening area of the gate for feeding the molten resin to the overflow portion be excessively small, the relation in adhering force between the transfer surface and the roughened surface would also be inverted and would thereby cause the mirror surface to sink. Should the opening area be excessively small, the molten resin would become short.
Prior art 1 can implement a mirror or similar optical element needing a single mirror surface because it roughens the mold surface facing the transfer surface. However, Prior Art 1 cannot produce a lens, prism or similar optical element because the number and positions of mirror surfaces are limited. In addition, the relation in adhering force is inverted and causes the mirror surface to sink, depending on the material constituting the transfer surface and roughened surface and the kind of the resin.
Prior Art 2 increases the cost of the mold assembly due to the porous member and sophisticates control over the configuration of the porous member. Specifically, if the effect of the porous member is excessive, it not only admits the molten metal thereinto, but also obstructs the parting of the molding. This is particularly true when the porous portion of the porous member extends inward over the wall of the mold. Further, because the compressed gas is fed to the non-transfer surface of the molding via the porous member during the previously stated interval, a pressure difference is maintained between the non-transfer surface and the transfer surface of the resin during cooling. As a result, the internal strain remains in the resulting molding after the opening of the mold. The residual pressure not only lowers the accuracy of the transfer surface, but also causes the entire molding to deform.
Prior Art 3 generates a pressure difference between the transfer surface and the non-transfer surface of the resin during the interval mentioned earlier. This also brings about the problem stated above in relation to Prior Art 2. Prior Art 4 maintains the transfer surface of the mold at a high temperature and heats the transfer surface side of the resin to a high temperature during the previously mentioned interval. This is also undesirable in the above respect.
It is therefore an object of the present invention to provide a method for producing a plastic molding capable of surely sinking only at a desired portion thereof and being surely provided with a mirror surface in another desired portion thereof.
It is another object of the present invention to provide an inexpensive and least deformable plastic molding capable of surely guiding sinking to a non-transfer surface thereof and therefore having a highly accurate transfer surface.
In accordance with the present invention, in a molding produced by an injection mold assembly having a pair of molds including a mold surface forming a cavity having a preselected volume, at least one transfer surface for transferring a mirror surface formed on the mold surface to the molding, and a gate for filling the cavity with a molten material by injection, and by injecting the molten material into the cavity via said gate and then cooling the molten material, the injection mold assembly includes at least one vent hole having a preselected opening area, and at least one bore communicated to the vent hole for applying a preselected air pressure to the molding. A step portion is formed on the mold surface between the vent hole and the transfer surface. Also, in accordance with the present invention, in an injection molding method for producing a molding by using a mold assembly having a pair of molds including a mold surface forming a cavity having a preselected volume, at least one transfer surface for transferring a mirror surface formed the mold surfaces to the molding, and a gate for filling the cavity with a molten material by injection, and by injecting the molten material into the cavity via the gate and then cooling the molten material, the mold surface is formed with, outside of the transfer surface, at least one vent hole having a preselected opening area and at least one bore communicated to the vent hole for applying a preselected air pressure to the molding material. The air pressure is continuously generated via the vent hole even after the pressure of the molding material in the cavity has dropped to zero.
Further, in accordance with the present invention, a mold assembly has a pair of molds including a mold surface forming a cavity having a preselected volume, at least one transfer surface for transferring a mirror surface formed on the mold surface to the molding, and a gate for filling the cavity with a molten material by injection, and injects the molten material into the cavity via the gate and then cools the molten material. The mold surface is formed with, outside of the transfer surface, at least one vent bole having a preselected opening area and at least one bore communicated to the vent hole for applying a preselected air pressure to the molding material, and at least one exhaust hole located at a position adjoining the vent hole, but not facing the transfer surface.
Moreover, in accordance with the present invention, a method of producing a plastic molding begins with the step of preparing a mold assembly including at least one transfer surface and at least one non-transfer surface formed on a surface other than the transfer surface. The transfer surface and non-transfer surface form at least one cavity. Molten resin heated to a temperature above a softening point thereof is injected into the cavity. A resin pressure is caused to act on the transfer surface to thereby cause the resin to adhere to the transfer surface, and then the resin is cooled to a temperature below the softening point. The mold assembly is opened in order to allow the resulting molding to be taken out. The temperature of at least one non-transfer surface of the resin is lowered below the temperature of the resin on the transfer surface during an interval between the beginning and the end of injection of the resin into the cavity.